The present invention relates to a thin film semiconductor, a process for producing a thin film semiconductor, a solar cell, and a light emitting diode. More particularly, it relates to a device and a method for forming a semiconductor film layer on a substrate having a plurality of porous layers defined therein having controlled and differing relative porosities.
As the material of the solar cell, various materials have been studied. Silicon, for which there are abundant reserves and which is free from apprehension of pollution, is the center of these efforts. Ninety percent or more of the amount of production of solar cells in the world are silicon solar cells as well. The tasks in solar cells are how to achieve a low cost, a high efficiency of conversion of light to electricity, a high reliability, and a small number of years for energy recovery. For the requests for high conversion efficiency and high reliability, single crystal silicon is most suitable, but it is difficult to fabricate single crystal silicon at a low cost. Therefore, at present, in the field of solar cells, particularly solar cells having a large surface area, active research and development is proceeding on solar cells using thin film polycrystalline silicon or thin film amorphous silicon.
In a thin film polycrystalline Si solar cell, the purity of the silicon is raised by refining techniques from metal class silicon using plasma or the like. An ingot is prepared by a casting process, and a wafer is prepared by a multi-wire or other high speed slicing technology. However, process for removing the boron and phosphorus from the metal class silicon, the preparation of an ingot of a good quality crystal by a casting process, enlargement of the surface area of the wafer, and a multi-wire or other high speed slicing technology require a very high grade of technology, so a substrate which is sufficiently cheap and has a good quality has not yet been fabricated at present. Further, the film thickness of the wafer is approximately 200 xcexcm, therefore a flexible substrate cannot be formed.
Amorphous silicon can be formed on the surface of a plastic substrate by a CVD (chemical vapor deposition) process. Therefore, it is possible to form flexible thin film amorphous silicon. As a result, solar cells having a wide range of applications can be formed. However, there are drawbacks in that the conversion efficiency is lower than that of the polycrystalline silicon and single crystal silicon, and the conversion efficiency deteriorates during use.
Single crystal silicon offers the promise of a high conversion efficiency and a high reliability. Thin film single crystal silicon can be fabricated by the SOI (Silicon On Insulator) technique, which is a manufacturing technique of integrated circuits etc., but the productivity is low. Using the SOI technique, the manufacturing cost becomes considerably high, this is a problem in application to a low cost solar cell. Further, the processing temperature in the preparation of single crystal silicon is relatively high, so it is difficult to form this on a plastic substrate or glass substrate having a low heat resistance. Since it is difficult to form single crystal silicon on a plastic substrate, the manufacture of flexible thin film single crystal silicon is difficult.
When constructing window glass equipped with solar cells, in other words, solar cells are arranged on a surface of a window glass, solar cars with solar cells arranged on the roof, etc., the use of a flexible solar cell is desirable from the viewpoint of the simplification of the manufacture and the ease of rational arrangement for enlarging the light receiving surface. Nevertheless, the only semiconductor silicon which can be used to make the flexible solar cells at the present time is amorphous silicon.
The present invention provides a method for making a thin film semiconductor with which a thin film semiconductor, for example, thin film single crystal silicon, can be reliably produced on a mass production basis and a device constructed in accordance with the method. Therefore, a reduction of cost can be achieved and processes for producing a solar cell and a light emitting diode with which a solar cell having a high opto-electric conversion efficiency can be reliably and easily produced at a low cost.
In an embodiment, the present invention provides a process for producing a solar cell with which the terminal of the solar cell can be easily and reliably led outside with a low resistance.
In an embodiment, a new and improved method for making a thin film semiconductor is provided comprising the steps of providing a semiconductor substrate having a surface. The surface portion of the substrate is treated in at least one anodization process to define a plurality of porous layers having varying degrees of porosity adjacent the surface. In a preferred embodiment, the substrate is anodized in a first anodization step at a first current density to provide a first porous layer adjacent the surface having a first porosity. A second anodization step is performed at a second, higher current density to provide a second porous layer adjacent the first porous layer opposite the surface having a porosity greater than the first porosity. The difference in porosity between the first porous layer and the second porous layer provides an inherent line or zone of relative weakness located in the second porous layer or at or adjacent to the interface between the first porous layer and the second porous layer. The line of weakness or fragility introduced by the strain caused by the difference in the lattice constants of the adjacent porous layers permits separation of the surface layer and any film grown thereon from the remainder of the second porous layer and the substrate. In an especially preferred embodiment, a third anodization step at a third higher current density is performed to define third porous layer having a third porosity higher then the second porosity. The third porous layer is disposed within the second porous layer or adjacent to the second porous layer. In accordance with this embodiment, a relative line of weakness is defined by the third porous layer or at or adjacent an interface formed between the third porous layer and the second porous layer. After a plurality of porous layers are defined adjacent the surface of the substrate, at least one semiconductor film layer is formed on the first porous layer and surface. Thereafter, the semiconductor film is separated from the semiconductor substrate along the line of relative weakness to provide a thin film semiconductor product.
According to the present invention, a thin film semiconductor is prepared by a changing a surface of a semiconductor substrate to form a porous structure comprising two or more porous layers having different porosities; growing a semiconductor film on the surface of the porous structure; and separating, removing and/or peeling the semiconductor film from the semiconductor substrate in a controlled or directed manner along the line of weakness created in the porous structure layers.
Further, in the process for producing a solar cell according to the present invention, a solar cell is produced by method comprising the steps of changing the surface of a semiconductor substrate to form a porous structure comprising two or more porous layers having different porosities; epitaxially growing a semiconductor film comprising multiple layers constituting the solar cell on the surface of this porous layer; and peeling or otherwise separating the multi-layer epitaxial semiconductor film from the semiconductor substrate along a line of weakness defined in the porous structure layers.
Furthermore, in a process for producing a light emitting diode according to the present invention, the light emitting diode is prepared by a method comprising the steps of changing the surface of a semiconductor substrate to form a porous structure layer comprising two or more porous layers including a first porous layer constituting a light emitting portion and a second porous separation layer, having a higher porosity, disposed beneath the light emitting portion and thereafter, peeling or separating the light emitting portion from the semiconductor substrate along the line of weakness defined in the separation layer of the porous structure.
As mentioned above, according to the process of the present invention, the semiconductor substrate surface per se is changed to form the porous layer, a semiconductor film is formed on the substrate by epitaxial growth, and this semiconductor film is peeled from the semiconductor substrate by breakage in the porous layer or at an interface with the porous layer, thereby the intended thin film semiconductor or solar cell is obtained. Accordingly, the epitaxially grown semiconductor film can be formed with any sufficiently thin thickness. Further, the peeling of the thin film semiconductor from the substrate can be reliably carried out by appropriately selecting the strength of the porous layer, for example, by the selecting the porosity in the porous layer. As described above, according to the present invention, a thin film semiconductor can be obtained with any sufficiently thin thickness with a good yield. Further, in the production of solar cells, a solar cell having a sufficiently high optoelectric conversion efficiency can be formed for the reasons that the active portion to be constituted by this epitaxial film can be constituted sufficiently thin and it can be formed by the single crystal thin film semiconductor layer, which is epitaxially grown. Further, since it is now possible to provide a flexible structure, various applications, for example, applications for window glass equipped with solar cells, solar car, etc. become easier to produce.
Further, in the light emitting diode embodiment according to the present invention, a superlattice structure can be formed by the porous layers having different porosities and therefore an improvement of the light emitting efficiency can be achieved.
Further in an embodiment, there is provided a method for making a thin film semi-conductor comprising the steps of: providing a semi-conductor substrate having a surface; anodizing the semi-conductor substrate to provide a first porous layer adjacent the surface having a first porosity; anodizing the semi-conductor substrate to provide at least one second porous layer adjacent the first porous layer opposite the surface, each said second porous layer having a second porosity greater than said first porosity; and separating an upper portion of the semi-conductor substrate from the semi-conductor substrate along a line of relative weakness defined in or adjacent one of said second porous layers.
In an embodiment, there is provided a method for making a thin film semi-conductor comprising the steps of: providing a semi-conductor substrate having a surface; anodizing the semi-conductor substrate at a first current density to provide a first porous layer adjacent the surface having a first porosity; anodizing the semi-conductor substrate at a second current density higher than said first current density to provide a second porous layer adjacent the first porous layer opposite the surface, the second porous layer having a second porosity greater than the first porosity; anodizing the semi-conductor substrate at a third current density higher than said second current density to provide a third porous layer in or adjacent the second porous layer, the third porous layer having a third porosity higher than said second porosity; and separating an upper portion of the semi-conductor substrate along a line of relative weakness defined in the third porous layer or at or adjacent an interface defined between said third porous layer and the second porous layer.
In an embodiment, there is provided a method for making a thin film semi-conductor comprising the steps of: providing a semi-conductor substrate having a surface; anodizing the semi-conductor substrate at a first current density to provide a first porous layer adjacent the surface having a first porosity; anodizing the semi-conductor substrate at a second current density higher than said first current density to provide a second porous layer adjacent the first porous layer opposite the surface, the second porous layer having a second porosity greater than the first porosity; anodizing the semi-conductor substrate at a third current density higher than said second current density to provide a third porous layer in or adjacent the second porous layer, the third porous layer having a third porosity higher than said second porosity; and separating an upper portion of the semi-conductor substrate from the semi-conductor substrate along a line of relative weakness defined in the third porous layer or at or adjacent an interface defined between said third porous layer and the second porous layer, wherein in said anodizing step, the semi-conductor substrate is contacted by an electrolytic solution and exposed to a flow of current at said first, second and third current density, respectively, and wherein in the anodizing steps, the electrolytic solution is the same.
In an embodiment, there is provided a method for making a thin film semi-conductor comprising the steps of: providing a semi-conductor substrate having a surface; anodizing the semi-conductor substrate at a first current density to provide a first porous layer adjacent the surface having a first porosity; anodizing the semi-conductor substrate at a second current density higher than said first current density to provide a second porous layer adjacent the first porous layer opposite the surface, the second porous layer having a second porosity greater than the first porosity; anodizing the semi-conductor substrate at a third current density higher than said current density to provide a third porous layer in or adjacent the second porous layer, the third porous layer having a third porosity higher than said second porosity; and separating an upper portion of the semi-conductor substrate from the semi-conductor substrate along a line of relative weakness defined in the third porous layer or at or adjacent an interface defined between said third porous layer and the second porous layer, wherein in said anodizing step, the semi-conductor substrate is contacted by an electrolytic solution and exposed to a flow of current at said first, second and third current density, respectively, and wherein the electrolytic solution used in the anodizing steps varies.
In an embodiment, there is provided a method for making a thin film semi-conductor comprising the steps of: providing a semi-conductor substrate having a surface; anodizing said semi-conductor substrate at a first current density to provide a first porous layer adjacent said surface having a first porosity; anodizing said semi-conductor substrate at a second current density higher than said first current density to provide a second porous layer adjacent said first porous layer opposite said surface, said second porous layer having a second porosity greater than said first porosity; annealing said semi-conductor substrate in a hydrogen atmosphere after said step of anodizing said semi-conductor substrate to provide said second porous layer.
In an embodiment, there is provided a method for making a thin film semi-conductor comprising the steps of: providing a semi-conductor substrate having a surface; forming a first porous layer adjacent said surface having a first porosity; forming a second porous layer within said first porous layer having a second porosity higher than said first porosity; and separating an upper portion of said semi-conductor substrate from said semi-conductor substrate along a line of relative weakness defined in or adjacent one of said first and second porous layers.
In an embodiment, there is provided a method for making a thin film semi-conductor comprising the steps of: forming a first porous layer having a first porosity on a surface of a substrate; forming a second porous layer within or underneath said first porous layer having a second porosity higher than said first porosity; and separating an upper portion of said semi-conductor substrate from said substrate along a line of relative weakness defined in or adjacent one of said first and second porous layers, wherein said first porous layer and said second porous layer are formed by anodizing.
In an embodiment, a thin film semi-conductor formed according to a method comprising the steps of: providing a semi-conductor substrate having a surface; forming a first porous layer having a first porosity on a surface of said substrate; forming a second porous layer within or underneath said first porous layer having a second porosity higher than said first porosity; and separating an upper portion of said semi-conductor substrate from said substrate along a line of relative weakness defined in or adjacent one of said first and second porous layers to obtain said thin film semi-conductor, wherein said first porous layer and said second porous layer are formed by anodizing.
Other objects and advantages of the present invention will become apparent from the following Detailed Description of the Preferred Embodiments taken in conjunction with the drawings, in which: